Electromechanical brakes have been known for some time. U.S. Pat. No. 5,788,023 discloses a disc brake for a vehicle which can be actuated electrically and whose brake linings can be pressed against the brake disc with the aid of an electric motor. The electric motor transmits its actuation force, via a so-called planetary rolling-contact threaded spindle, onto an axially displaceably mounted piston which interacts with the brake lining.
U.S. Pat. No. 5,829,557 discloses another vehicle disc brake which can be actuated electrically and whose brake linings can in turn be pressed against the brake disc by means of an electric motor serving as an actuator. The electric motor comprises a spindle gear mechanism and, by means of a spindle element which can be of different designs, is connected, in the direction of displacement of the brake linings, to an axially displaceable piston which acts on a brake lining. In this patent, there is optional provision for the use of an additional gear mechanism for converting the torque and rotational speed.
A major problem with conventional brakes with an electric actuator is the high actuator force that has to be applied in order to achieve a sufficient braking effect. The necessary high actuator force and the resulting large power demand of the actuator make it necessary to employ very large drive units, usually electric motors, which have large torques, and are also heavy and expensive. The result of this is that electromechanical brakes have, to date, not become widespread as vehicle brakes, for example.
In order to decrease the energy consumption of the brake actuators, so-called self-enforcing actuators have been proposed. Early examples of such self-enforcing brakes can be found in U.S. Pat. Nos. 4,653,614, 4,852,699, 4,946,007, 4,974,704, 5,012,901. A self-enforcing brake works according to the principle that the braking force amplifies itself. The friction force between the brake linings and the brake disc give rise, with help of a self-enforcing mechanism, to increased force against the brake linings and brake disc. This increased force gives, in turn, rise to increased friction force. Hence, it is possible to produce and control large braking forces by applying relatively moderate forces.
The degree of self-enforcement defines the relation between the applied force and the actual braking force. The self-enforcement is strongly dependent on the disc/pad friction coefficient. Normally, the variations in the disc/pad friction coefficient are large, and are dependent on, among other factors, the temperature of the disc and/or pad. Variations in disc/pad friction coefficient are even possible within one and the same brake application.
At a specific disc/pad friction coefficient, μinf, the static reinforcement of the self-enforcing mechanism is principally infinity. That means that one can produce and control large brake forces by only applying relatively moderate forces. For disc/pad friction coefficients lower than this specific number, the brake is stable, which means that a pushing force has to be applied to produce brake forces. For disc/pad friction coefficients larger than μinf, the self-enforced brake instead will become unstable, which means that a pulling force has to be applied to hold the brake at a specific brake force or else uncontrolled braking (i.e., lockup) can occur.
Thus, it should be recognized that while proper control of all electromechanical brake actuators is important, proper and accurate control at all times of self-enforcing brake actuators is even more critical so as to avoid uncontrolled braking with possibly catastrophic results.
One of the ways in which failure can occur in such systems is if there is a failure within the control network that controls actuation of the brake actuators, or within the brake actuators themselves. In order to deal with such situations, and in attempts to provide a fail safe brake system, it has been proposed to provide redundancy in certain aspects of the braking system.
U.S. Published Patent Application No. US 2005/0127749 A1 (hereinafter referred to as “the '749 application”) discloses such a system, in which is provided at least one central control unit (72) and a control means (84) associated with each braking module (66). Each control means (84) includes a main control unit (80) which is in communication with the central control unit (72) and with the various system sensors, and an auxiliary control unit (82) which is in communication with the main control unit (80). The main control unit (80) controls operation of drives (34, 34′) of the self-enforcing brake (10), while the auxiliary control unit (82) controls operation of an adjusting means (42) of the self-enforcing brake (10).
While the system disclosed in the '749 application does provide for some separation of control duties between the main control unit (80) and the auxiliary control unit (82), and does provide for some redundancy in certain respects, it does not provide true redundancy of brake actuation and suffers from serious disadvantages. One of such disadvantages is that only the main control unit (80) is in direct communication with the central control unit (72) and with the various system sensors; the auxiliary control unit (82) communicates with these elements only through the main control unit (80). Thus, in the event that the main control unit (80) ceases to function properly (due to a power failure, a short, physical damage, or for any of a number of other reasons), the auxiliary control unit (82) may be isolated from the central control unit (72) and the various system sensors, thereby severely restricting operation of the auxiliary control unit (82), or even rendering the auxiliary control unit (82) essentially useless.
Another disadvantage of the system described in the '749 application is that the auxiliary control unit (82) is capable of only very limited brake actuation, even when the auxiliary control unit (82) is fully functional. True brake actuation control capability is not provided. Instead, again assuming that the main control unit (80) ceases to function properly (due to a power failure, a short, physical damage, or for any of a number of other reasons) and is no longer able to properly control drives (34, 34′), the auxiliary control unit (82) allows only for control of the adjusting means (42). As the '749 application itself recognizes, this provides only very limited control of the self-enforcing brake (10), permitting an open brake to remain open, and possibly permitting a closed brake to be caused to open. The auxiliary control unit (82) can not be used to control adjusting means (42) such that the self-enforcing brake is caused to be placed or remain under load, or there would exist the very real possibility of uncontrolled braking (i.e., lockup) due to the nature of the self-enforcing brake (10).
What is desired, therefore, is a system for controlling application of an electronically controlled brake which is well-suited for controlling the application of an electromechanical brake unit with self-energizing characteristics, which includes a redundant brake actuator for providing fail safe operation, which provides redundant communication with at least one central control unit and system sensors such that a functioning portion of the brake actuator is still in communication with these elements even during failure of another portion of the brake actuator, and which allows for a functioning portion of the brake actuator to provide at least a level of brake actuator control sufficient to avoid catastrophic consequences even during failure of another portion of the brake actuator.